Such a photonic mixer is known from EP-A 1513202. Such a photonic mixer is for instance intended for “time of flight” (TOF) range-finding applications. Prior art documents WO98/10255 and WO99/60629 explain the general principle of time of flight measurements for range finding applications. A light source is modulated at a frequency in the range of 1 MHz to 1 GHz. The light illuminates an object, scene and part of the reflected light enters the range finder camera through a focused lens. By measuring in each pixel the phase of the incident light, a distance can be estimated between the pixel and its conjugate (light-reflecting) pixel area in the scene. In this way the distances of objects and shape of objects can be estimated and recorded.
The photonic mixer is a device for measuring the phase of the reflected light accurately. This phase accuracy is very important, since it is linked to the precision of estimating the distance. The photonic mixer mixes the incident light right away in the detector, instead of in a connected electronic mixer. Therewith, low noise is achieved, and thus a better signal to noise ratio and a smaller error on the estimated distance. Particularly relevant in the operation of the photonic mixer is that the electric signal is applied to the substrate with the same modulation frequency as the electromagnetic radiation. In this manner, the mixing of the signal occurs directly in the substrate. In traditional photodetectors, the mixing is carried out at a later stage in a separate mixer.
FIG. 6A illustrates an embodiment of the prior art photonic mixer, that mixes incident amplitude modulated electromagnetic radiation with an electrical signal applied to the substrate 1—typically an epi-layer thereof—through source 90. FIG. 6B shows a cross-section through line III-III′ of the device illustrated in FIG. 6A. The electrical signal applied by source 90 generates a majority current, e.g. majority hole current 99, through the substrate 1. The applied electrical signal is typically modulated with the same frequency as the electromagnetic radiation, typically light. When photons from the electromagnetic radiation impinging on the field area of substrate enter the field in the substrate, these are converted into pairs of electrons and holes. Minority carriers, like electrons in a p-substrate 1 will feel the electric field that is associated with the applied majority hole current 99, and will drift towards a first source of majority carriers, e.g. holes, which is in the example illustrated p+-contact region 61. They will then diffuse into an adjacent first detector region comprising a well or collector region 67 and a contact region 63. In this manner, they will become part of an output photocurrent of the left mixer connection point Mix1. A possible electron trajectory is trajectory 66, as illustrated in FIG. 6B. When the applied voltage or electrical signal is inverted as illustrated in FIG. 6C, the direction of the majority current flow is inverted, and the minority carriers drift towards the complementary contact region, p+-finger 62. Thereafter, most of the minority carriers, namely electrons, diffuse into the second detector region 64 via n-well 68, becoming part of an output photocurrent of the right mixer connection point MIX2. A possible electron trajectory is trajectory 69, as shown in FIG. 6C. In this way, the field area sensitive to incident photons becomes large, whilst the detector regions 63, 64 have only a small capacitance due to their limited finger areas. Electromagnetic masking such as metal regions 60 can be used to prevent that the impinging electromagnetic radiation, such as light penetrates in unwanted areas.
It has been observed in experiments with the prior art photonic mixer that the obtained modulation frequency was clearly less than the theoretically calculated maximum. This has the disadvantage that the bandwidth of the photonic mixer is limited. The bandwidth limitation not merely limits overall speed of the device, but also imposes a limit to its precision.
It is therefore an object of the invention to provide a photonic mixer of the kind mentioned in the opening paragraph that is suitable for use at increased modulation frequencies, such as above 10 MHz, more specifically above 100 MHz.